Gas sensors are employed in a variety of applications requiring qualitative and quantitative gaseous determinations. In the automotive industry, it is well known that the oxygen concentration in the automobile exhaust has a direct relationship to the engine air-to-fuel ratio. Oxygen gas sensors are employed within the automobile internal combustion control system to provide accurate exhaust gas oxygen concentration measurements for determination of optimum combustion conditions, maximization of efficient fuel usage, and management of exhaust emissions.
Generally, the electrochemical type of oxygen sensor employed in automotive applications utilizes a thimble shaped electrochemical galvanic cell to determine, or sense, the relative amounts of oxygen present in the exhaust stream, an example being U.S. Pat. No. 3,844,920 to Burgett et al. This type of oxygen sensor is generally known and used throughout the automotive industry, and comprises an ionically conductive solid electrolyte material, typically yttria stabilized zirconia, a porous electrode coating on the exterior exposed to the exhaust or measuring gas and a porous electrode coating on the interior exposed to a known concentration of reference gas.
The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes by the Nernst equation: E=AT Ln[P.sub.1 /P.sub.2 ], where E is the galvanic voltage, T is the absolute temperature of the gas, P.sub.1 /P.sub.2 is the ratio of the partial pressures of the reference gas at the two electrodes, and A=R/4F, where R is the universal gas constant and F is the Faraday constant. Thus, the oxygen sensor senses the oxygen concentration in the exhaust gas by measuring this galvanic output voltage.
However, it is desirable to first heat the solid electrolyte of such a sensor to an elevated temperature. This enables the sensor to immediately obtain an appreciable output voltage in response to the difference in the oxygen concentrations between the reference and measuring electrodes. The induced galvanic potential between electrodes and corresponding output voltage are not stable until the solid electrolyte has been heated to a given temperature. In operation, the combustion gases heat the solid electrolyte of the oxygen sensor to an operating temperature sufficient to effect galvanic stability. Therefore, effective sensor operation is delayed until the combustion gases heat the sensor to a suitable temperature.
Lastly, if the sensor is placed too far downstream in the exhaust pipe of an engine, especially a highly efficient engine, the sensor may not be heated to a high enough temperature during engine idle to meet sensor specifications. During these conditions, the internal combustion engine control system operates open loop, i.e., the control system does not sense the controlled parameter, air-to-fuel ratio, in order to control that parameter. It is known that a large percentage of the total emissions produced during short periods of operation are produced during this period, engine warm up. Therefore, in some applications, emissions control during engine warm up would be improved with an oxygen sensor which had means for rapidly heating itself to a predetermined temperature, regardless of the temperature of the surrounding environment. Also desirable about an oxygen sensor which can heat itself is that it may be placed anywhere in the exhaust pipe, even at the cooler exit end, since the solid electrolyte of the sensor is not dependent on the heat of the combustion gases for heating itself.
Many heated oxygen sensors have been previously proposed in the art. These prior heated oxygen sensors generally comprise an elongated ceramic heater which positively heats the solid electrolyte body of the sensor. The heater element is typically inserted into an elongated cylindrical hole formed in the conventional thimble shaped solid electrolyte body.
Although these prior types of heated and unheated oxygen sensors have performed satisfactorily during operation, it is desirable to develop a sensing element which is not thimble shaped. A shortcoming of the oxygen sensors having the thimble shaped elements are that they are accordingly difficult to manufacture and assemble.
It is therefore desirable to provide a sensing element which utilizes a flat plate ceramic substrate and which utilizes standard ceramic processing techniques. In addition, it is also desirable that such a sensing element be capable of rapidly heating itself to a predetermined temperature. Further, for automotive applications particularly, the heated oxygen sensing element should be rugged, reliable, and readily manufacturable at a low cost.